Without limiting the scope of the invention, its background is described in connection with microfluidic structures. Current industry approaches for arraying fluid volumes involve expensive methods such as robotic dispensing in micro-scale multi-well plates or spotting of fluids on substrates. These approaches suffer from using microliter-scale fluid volumes, and are also not amenable to further reduction in fluid volumes due to liquid evaporation. Moreover, these approaches do not allow isolation and study of individual cells, which is important in applications such as cancer and bacterial infections, where there is a need to identify the ‘rogue’ cells among a population.
Droplet-based microfluidics has the potential to offer flexible and cheaper approaches to alleviate the problems associated with current robotic dispensing systems. Recent advances in droplet-based microfluidics1-3 have provided a unique paradigm to compartmentalize reactions in very small volumes for applications ranging from biomolecule analysis4-11 to cell-based assays12-21 to fabrication of novel materials22-26. Monodisperse nanoliter- to picoliter-scale drops can be rapidly generated in an immiscible carrier phase using pressure-driven flows in microfluidic devices27-29. Additional benefits of such droplet-based microfluidics include the ability to isolate single biomolecules or cells in drops and minimize product dispersion during a reaction. There remains, however, a need for structures and methods that allow for isolation and study of, e.g., individual cells or biomolecules.